Obstructive sleep apnea syndrome (OSAS) is a well-recognized disorder which may affect as much as 1-5% of the adult population. OSAS is one of the most common causes of excessive daytime somnolence and is most frequent in obese males. It is the single most frequent reason for referral to sleep disorder clinics. OSAS is associated with all conditions in which there is an anatomic or functional narrowing of a patient's upper airway, and is characterized by an intermittent obstruction of the upper airway during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia and sleep disruption associated with the apneas and arousals from sleep.
The pathophysiology of OSAS has not been fully worked out. However, it is now well recognized that obstruction of the upper airway during sleep is in part due to the collapsible behavior of the supraglottic segment of the airway during the negative intraluminal pressure generated by inspiratory effort. Thus, during sleep, the upper airway behaves substantially as a Starling resistor. That is, the flow through the upper airway is limited to a fixed value irrespective of the driving (inspiratory) pressure. With the loss of airway tone characteristic of the onset of sleep and which may be exaggerated in OSAS, partial or complete airway collapse may occur.
Since 1981, positive airway pressure (PAP) applied by a tight fitting nasal mask worn during sleep has evolved as the most effective treatment for this disorder. The availability of this non-invasive form of therapy has resulted in extensive publicity for apnea and the appearance of large numbers of patients who previously may have avoided the medical establishment because of the fear of intubation and/or tracheostomy. Increasing the comfort of the system (e.g., comfortable masks, minimizing applied nasal pressure, etc.) to improve patient compliance with therapy has been a major goal of research.
PAP therapy has become the mainstay of treatment in Obstructive Sleep Disordered Breathing (OSDB), which includes Obstructive Sleep Apnea, Upper Airway Resistance Syndrome, Snoring, exaggerations of sleep induced rises in collapsibility of the upper airway and all conditions in which inappropriate collapsing of a segment of the upper airway causes significant obstruction to airflow. This collapse generally occurs whenever pressure in the collapsible portion of the airway becomes sub-atmospheric or, more accurately, becomes lower than a “tissue pressure” in the surrounding wall. PAP therapy is directed to maintaining pressure in the collapsible portion of the airway at or above the critical “tissue pressure” at all times. This goal has previously been achieved by raising the airway pressure in the entire respiratory system to a level higher than this critical pressure.
Despite its success, limitations to the use of PAP still exist. Most limitations take the form of discomfort from the mask and the nasal pressure required to obliterate the apneas. Determining the minimum effective pressure remains a goal of the preliminary testing of a patient in the sleep laboratory. However, it has been shown that this required pressure varies throughout the night with sleep stage and body position. Furthermore, the therapeutic pressure may both rise and fall over time in patients with changing anatomy (e.g., nasal congestion/polyps), changes in weight, changing medication or with alcohol use. Consequently, most sleep laboratories currently prescribe the setting for home use of nasal PAP based upon the single highest value of pressure needed to obliterate apneas during a night of monitoring in the sleep laboratory. Retesting may often be necessary if the patient complains of incomplete resolution of daytime sleepiness, and may reveal a change in the required pressure.
There are also limitations to the diagnosis of OSAS which requires monitoring airflow at the nose and mouth. Algorithms are available for the diagnostic criteria using a nasal cannula/pressure traducer system to detect characteristic changes in the nasal airflow system which identify airflow obstruction (i.e., flow limitation events). In particular, a diagnostic study must be performed before the nasal PAP is prescribed. This diagnosis may include a titration study in a laboratory or the use of an autotitration device. However, these procedures may only be justified after a preliminary diagnostic study has been performed.
In many OSAS treatment centers, diagnosis and initiation of the PAP therapy are frequently combined into a single night of monitoring a patient. For example, current Medicare guidelines require a two hour minimum of monitoring in a diagnostic mode to complete the diagnosis of OSAS. The diagnosis may also be performed during ambulatory and/or unattended monitoring. However, the benefit of the unattended monitoring decreases if, for example, a second night and separate equipment are needed to initiate the PAP therapy.
Although there are some “smart” PAP systems that adjust their applied pressure based on various criteria, these systems cannot decrease their pressure to “zero” pressure due to the potential for the build up of excessive levels of CO2. Certain PAP systems used in scientific research allow for zero pressure. However, these systems are suitable only for diagnosis and are not capable of delivering therapeutic pressures.